Thermally anisotropic sheets whose thermal conductivity in their surfacewise directions (hereinafter referred to as “thermal conductivity in parallel direction”) is higher than that in their thicknesswise directions (hereinafter referred to as “thermal conductivity in perpendicular direction”) have been used to conduct heat from heat sources to other places. The higher the thermal conductivity in parallel direction of a sheet is, the quicker the thermal conduction through it is; accordingly, sheets of high thermal conductivity in parallel direction have been developed.
Disclosed in the Japanese Unexamined Patent Publication No. 2001-79977 is a graphite sheet of high thermal conductivity in parallel direction. This graphite sheet is made by heat-treating a film of a polymer such as polyimide over 2,400° C., and its thermal conductivity in parallel direction is as high as 500-800 W/(m·K). However, such polymer films are costly. Besides, heat-treating of such polymer films takes a long time; therefore, the production efficiency of the graphite sheet is low. Accordingly, the graphite sheet is very costly and, hence, the application of the graphite sheet to apparatuses and equipment is limited.
Although the thermal conductivity in parallel direction of the graphite sheet of the above Japanese Unexamined Patent Publication No. 2001-79977 is very high, its thermal conductivity in parallel direction is not uniform if its thickness or density is not uniform. When it is used to conduct heat from a heat source to other places, spots of low thermal conductivity becomes heat spots which affect nearby components or devices. However, no method of preventing such heat spots from being made is disclosed in the Japanese Unexamined Patent Publication No. 2001-79977.